This invention relates to polyacetal compositions having good wear resistance/sliding properties and also having good melt processing thermal stability. More specifically, it relates to polyacetal compositions containing at least one lubricant and at least one ethylene-based polymer.
Polyacetal resins have been widely used in various sliding applications (such as bearing materials, gears, cams, conveyor chains, etc.). It is known that various solid and liquid lubricants, such as molybdenum disulfide, graphite, polytetrafluoroethylene, paraffin oil, fatty ester, silicone etc., are added into polyacetal resins to improve friction and wear properties in such sliding applications. However, it was found that solid lubricants did not achieve desired results and that liquid lubricants (as in, for example, U.S. Pat. No. 3,808,133) caused an increase in screw retraction time due to slippage occurring during molding and extrusion processes.
To improve the performance of polyacetal in sliding applications, polyethylene and polypropylene were added to polyacetal (U.S. Pat. No. 4,945,126). However, the resultant compositions frequently suffered from peeling or delamination on the surface of molded parts and further had poor heat stability. The inclusion of lubricants and ethylene vinyl acetate copolymer into polyacetal improved friction and wear properties of polyacetal (U.S. Pat. No. 4,041,002). However, such compositions were found to have poor heat stability since the acetate groups of the ethylene vinyl acetate copolymer were found to destabilize polyacetal chains.
As such, there still exists a need to develop a polyacetal composition having not only good wear resistance properties, but also having good heat stability properties.